Music therapy, a frequently used arts-based therapy, has become standard palliative care in many pediatric and adult hospitals; however, few studies have examined mechanisms by which music therapy interventions work. Based on the Contextual Support Model of Music Therapy, we developed and tested the Active Music Engagement (AME) intervention, establishing it as a feasible and acceptable intervention that reduces emotional distress in young children (ages 3-8) hospitalized for cancer treatment. Emotional distress in young children with cancer during acute treatment and their parents is a prevalent, persistent problem associated with physical symptom distress and diminished quality of life and family function. The music therapist-led AME uses music-based play and parent education/support (music play resource kit; tip sheets), is easy to implement, and teaches parents/children how to therapeutically use a familiar activity to manage distress. The purpose of this two group randomized controlled trial is to identify behavioral, sociological, and psychological variables contributing to positive outcomes observed in previous AME studies (i.e., mediators) and identify for whom the intervention works (i.e., moderators). We will examine proximal mediators of child engagement and parent- child interaction and distal mediators of perceived family normalcy, parent confidence (self-efficacy) about their ability to support their child during treatment, and independent parent/child use of music play to manage distress during hospitalization. We hypothesize these factors mediate change in outcomes of child emotional distress, physical symptom distress, and quality of life; parent emotional/ traumatic distress and quality of life; and family function. Specific aims are to examine: 1) effects of proximal and distal mediators of AME on young child/parent outcomes; 2) moderators of AME on young child/parent distress; 3) explore child physical symptom distress (pain, fatigue, nausea) in mediation and moderation models. Child/parent dyads (n=184) will be stratified by age and randomized in blocks of 6 to AME or attention control; each group will receive three 45-minute sessions with a credentialed music therapist for 3 consecutive days with data collection at baseline, post-intervention, and 30 days later. Mediation effects will be estimated using ANCOVA, fitting appropriate mediation models using MPlus and then testing indirect effects using the percentile bootstrap approach to estimate indirect effect. Moderation effects will be tested by including appropriate interaction terms of the potential moderator with the intervention indicator in our models. Subsequent studies will examine AME across repeated hospitalizations to manage distress and prevent traumatic stress symptoms in survivorship.